1. Field of the Invention
The present invention relates to an xe2x80x98on-chipxe2x80x99 higher-to-lower voltage input stage. More particularly, the present invention relates to an input stage, incorporated as part of a monolithic integrated circuit, that is capable of carrying out voltage level conversion where the input voltage to the input stage is capable of exceeding at least one of the voltages applied to the input stage""s voltage supply rails.
2. Background Art
FIG. 1 illustrates a known circuit for translating one voltage to another voltage.
This circuit 100 comprises two n-type MOS transistors MN1 and MN2 operatively arranged to form what is commonly referred to as a xe2x80x98source followerxe2x80x99; so called since the voltage appearing on the source terminal of transistor MN1 tracks, or follows, that which is applied to its gate terminal. Transistor MN1 has its drain terminal connected to a positive supply rail VDD, its source terminal 110 is connected to the drain and gate terminals of transistor MN2 and its gate control terminal 120 receives a first voltage V1. Transistor MN2 has its source connected to the supply rail VSS. An output voltage Vout, which is derived from the first voltage V1, appears at the common connection 110 between transistors MN1 and MN2. Transistor MN2 is a diode connected transistor and therefore acts as an active resistor, alternatively transistor MN2 can be considered as acting as a current source.
A circuit 100 such as that shown in FIG. 1 is used in analogue circuit designs and one application is a voltage divider. If such a circuit 100 were to be used as an input stage of an integrated circuit it would incorporate additional circuitry for protection against Electro-Static Discharge (ESD): such ESD protection shall be described in more detail in relation to FIG. 2.
FIG. 2 illustrates a known CMOS inverting stage 200.
The inverting stage 200, including its Electro-Static Discharge protection diodes D1 and D2, which would typically be seen at an input pin of a digital integrated circuit (not illustrated), comprises p-type and n-type transistors MP3 and MN3.
Transistor MP3 has its source terminal connected to a positive supply rail VDD, its respective drain and gate terminals 210, 220 are connected to the respective drain and gate terminals of transistor MN3. Transistor MN3 has its source connected to the supply rail VSS. The respective input and output voltages Vin, Vout of the stage appear at the respective common gate and drain terminals 220, 210 of transistors MP3 and MN3.
Diode D1 has its anode connected to the supply rail VSS and its anode connected to the gate terminal 220. Diode D2 has its anode connected to the gate terminal 220 and its cathode connected to a supply rail VDD.
It should be noted that when the input voltage Vin exceeds the supply voltage VDD, diode D2 would act to clamp the input voltage Vin to a value of approximately VDD+VD: where VD is the forward voltage drop of a diode. Such clamping would be an undesirable effect and disadvantageous to an xe2x80x98on-chipxe2x80x99 higher-to-lower voltage input stage.
As the semiconductor process technologies advance the reduction in the geometry""s of transistors, and hence the overall size of integrated circuits, also leads to a reduction in the supply voltages which in turn leads to lower power, more efficient electronic circuits, systems and apparatus. For a number of years the supply voltage for many integrated circuits remained, and still remain, at 5 volts. However, due to the advances in process technology these supply voltages are being driven down to lower values. For example, 5 volt CMOS circuits are being replaced with circuits that operate on approximately 2 and 3 volt technology. Therefore, there is a need for an xe2x80x98on-chipxe2x80x99 higher-to-lower voltage input stage that will allow 5 volt and 3 volt technology, for example, to be interfaced without the need of costly external circuits and components.
Therefore, due to the aforementioned disadvantage and associated problems in relation to a need for an interface between such 2/3 volt and 5 volt technologies, for example, solutions have been proposed. One such proposal in the form of a circuit which is taught in the U.S. Pat. No. 5,151,619 to Austin et al., which is herein incorporated by reference. However, there is an associated problem associated with the circuit of U.S. Pat. No. 5,151,619 in that it is an xe2x80x98off-chipxe2x80x99 solution and as such it increases the component count and complexity, size and the cost of a system employing such an arrangement.
Accordingly, an object of the present invention is to provide circuitry that overcomes the aforementioned problems and/or disadvantages.
Another object of the present invention is to provide a circuit that is tolerant of an input voltage that exceeds at least one of its voltage supply rails.
Another object of the present invention is to provide a circuit that can be used as a x to y volt level translator, where the magnitude of x is greater than that of y.
Another object of the present invention is to provide a level translator circuit that is input voltage tolerant and that can be incorporated within a monolithic analogue and/or digital integrated circuit.
In order to achieve these objects, the present invention proposes an input stage of an integrated circuit that comprises: first and second voltage dividers 305, 305xe2x80x2; and a comparator325; said first and second voltage dividers, which are operatively connected between second positive and negative voltage supply rails VDD, VSS, respectively receiving an input and reference voltage Vin, Vref and respectively providing first and second outputs voltages Vout 1, Vout 2, said output voltages being input to the comparator, which is operatively connected between said second voltage rails, for providing a third output voltage Vout3, said input voltage being supplied on an input terminal 320 by first circuitry 330, that is supplied from first positive and negative voltage supply rails VH, CL, wherein the input voltage can pass beyond a voltage applied to at least one of the second voltage supply rails.
According to another embodiment of the present invention, the voltage dividers each comprise an MOS type transistor and a current source CS1, CS1xe2x80x2 that are operatively connected in series between said second positive and negative voltage supply rails, the gate 320 of said transistor of said first voltage divider being responsive to the input voltage, the gate 320xe2x80x2 of said transistor of said second voltage divider being responsive to the reference voltage, the current sources of said respective first and second voltage dividers being responsive to their respective output voltages.
According to another embodiment of the present invention the input terminal is operatively connected to second circuitry D1, D2 for operatively protecting the input stage against electrostatic discharge, said second circuitry being operatively connected between the first positive voltage supply rail and the second negative voltage supply rail.
According to another embodiments of the present invention the first and second negative voltage supply rails are connected together and the MOS transistors MN1, MN1xe2x80x2 are n-type MOS transistors having their drain terminals connected to the second positive voltage supply rail and their source terminals connected to the second negative voltage supply rail via their respective first and second current sources.
According to another embodiment of the present invention the input voltage is capable of increasing beyond a voltage applied to the second positive voltage supply rail.
According to another embodiment OLD the present invention the first and second negative voltage supply rails are at a voltage substantially equal to the ground potential of the input stage and the first and second circuitry and the second positive voltage supply rail is at a voltage less than +5 volts but greater than the voltage of the first and second negative voltage supply rail.
According to another embodiment of the present invention the second positive voltage supply rail has an applied voltage substantially in the range of +1 volt to +4 volts.
According to another embodiment of the present invention the first and second current sources are replaced by first and second resistive elements.
According to other embodiments of the present invention the resistive element is a passive and/or active resistive element.
According to other embodiments of the present invention the active resistive element is an operatively n-type diode connected MOS transistor MN2.
According to other embodiments of the present invention the integrated circuit incorporating an input stage according to the present invention is used in a system or apparatus that is incorporated within, or that is used in conjunction with, a computer; a domestic or consumer appliance; a vehicle; or a telephone or a telephone network.